A supermassive black hole (SMBH) likely resides at the center of the Milky Way, and in the centers of other galaxies like it. It’s never been seen though. It was discovered by watching a cluster of stars near the galactic center, called S stars.

S stars’ motions indicated the presence of a massive object in the Milky Way’s center and the scientific community mostly agreed that it must be an SMBH. It’s named Sagittarius A*.

But some scientists wonder if it really is a black hole. And one of the S stars could answer that question and a few others about black holes.

Scientists have been monitoring and studying the S stars for over 20 years. They’ve gathered precise astrometric data for the group of stars, and the measurements of the stars’ positions and movements around the galactic center have shown that there’s a massive object there. One particular star in the group—named S2 (or S0-2)—could help astronomers determine more clearly the nature of that massive object.

A new research letter is taking a close look at S2’s behaviour and asking a potentially uncomfortable question. The study is titled “What does lie at the Milky Way centre? Insights from the S2 star orbit precession.” It’s available on the pre-print site arxiv.org. The first author is C. R. Argüelles from the Fac. de Ciencias Astron. y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque.

The question of what’s at the Milky Way’s Center has largely been settled. The 2020 Nobel Prize in Physics was awarded to three scientists. Two of them, Andrea Ghez and Reinhard Genzel, received it for their research into the object at the center of the galaxy. The press release from the Nobel Prize Organization reads “Reinhard Genzel and Andrea Ghez discovered that an invisible and extremely heavy object governs the orbits of stars at the centre of our galaxy. A supermassive black hole is the only currently known explanation.”

Artist’s impression of the orbits of three of the stars very close to the supermassive black hole at the center of the Milky Way. Credit: ESO/M. Parsa/L. Calçada

Now there’s some uncertainty.

It’s possible that the massive object at the center of our galaxy is actually dark matter. In this case, fermionic dark matter. According to the authors, “It has been recently demonstrated that both, a classical Schwarzschild black hole (BH), and a dense concentration of self-gravitating fermionic dark matter (DM) placed at the Galaxy centre, can explain the precise astrometric data (positions and radial velocities) of the S-stars orbiting Sgr A*.”

There’s another cosmic actor playing a role in this question. It’s not a star, but a cloud of gas called G2. In 2014 G2 passed close by the Milky Way’s center. Astronomers think it came to within 36 light hours of Sgr A*. Prior to its closest approach, astronomers simulated what the encounter might look like. They thought that the black hole would tear gas from the cloud, leading to a pronounced brightening from Sgr A* as it accreted mass from G2. But it never happened.

This simulation of a gas cloud passing close to the supermassive black hole at the centre of the galaxy shows the situation in mid-2013. Observations with ESO’s Very Large Telescope confirm that the cloud is now so stretched that the front part of it has passed the closest point and is travelling away from the black hole at more than 10 million km/h, whilst the tail is still falling towards it.
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